1. Field of the Invention
The present invention relates to a method for manufacturing a paste for forming a thick film, the paste for forming the thick film and a filter apparatus. In particular, the present invention relates to a method for manufacturing a conductive paste for an internal electrode of a monolithic ceramic capacitor, etc., the conductive paste produced by the aforementioned method for manufacture and a filter apparatus.
2. Description of the Related Art
Pastes for forming a thick film have various solid materials depending on the purpose dispersed in organic vehicles. A paste for forming the thick film means a paste used in the formation of the thick film by a screen printing, a spray method, etc. More specifically, various pastes, for example, conductive pastes for forming internal electrodes of laminated ceramic electronic components, resistor pastes for forming thick film resistances of thick film circuit substrates, dielectric material pastes for forming monolithic ceramic capacitors and insulator pastes for insulation between layers of multilayer wiring substrates, are examples.
For example, as the conductive paste for forming internal electrodes of monolithic ceramic capacitors, etc., the pastes have noble metal powders, e.g., Au, Pd, Ag and Ag/Pd alloys, and base metal powders, e.g., Ni and Cu, as solid materials, dispersed in organic vehicles with a triple roller mill, etc., and may be filtrated using filter parts such as stainless steel wire gauzes (sieves) and filter cloths having mesh opening sizes of about 40 μm. The stainless steel wire gauze used for filtering the dispersion paste is composed of, for example, stainless steel wires knitted into the shape of a mesh, and foreign materials larger than the mesh opening size can be removed by passing the paste through the mesh part of the stainless steel wire gauze.
According to the conventional methods for manufacturing the paste for forming the thick film, however, protrusions of several μm to about 40 μm are likely to be generated on the print surface printed with the paste. It is believed that the protrusions are generated by solid materials of several μm to about 1 μm, metal foils generated in the triple roller mills, dusts which get into the paste during other steps, impurities such as skins, and insoluble materials in resin components, such as ethyl cellulose and acryl, contained in the organic vehicle (hereafter these are generically called lump materials), in the paste for forming the thick film. It is difficult to sufficiently remove those lump materials with the aforementioned stainless steel wire gauzes and filter cloths.
During filtration with the conventional stainless steel wire gauzes and filter cloths, lump materials having a size in the neighborhood of 1.4 times to 2 times the nominal mesh opening size pass through in practice. In particular, since the mesh openings are planar, needle-like lump materials, etc., are likely to pass through the conventional filtration units. When a pressure was applied to the materials to be filtrated, some lump materials are deformed due to this pressure so as to pass through the filter part.
When the paste for forming thick film is a conductive paste, and the internal electrodes of the laminated ceramic electronic component were formed using this, the conductive paste may not pass through the print screen mesh during the printing on the ceramic green sheet when undesired sizes of lump materials are present in the conductive paste, and therefore, there were problems in that deficiency was generated in the printed pattern, etc.
Furthermore, in the case in which the thickness of the ceramic green sheet was smaller than the size of the aforementioned lump materials, the aforementioned lump materials penetrated the ceramic green sheet, so that there were problems in that the reliability and the yield of the resulting electronic component were reduced by a large degree.
In order to improve the printing precision of the paste for forming thick film used in the screen printing, the viscosity was made relatively high, e.g., 1 to 50 Pa·s, and a thixotropic property was provided. But since the solid materials had small degree of freedom of movement in the dispersion paste, and the conventional high filtration precision filter had a large pressure loss, there was a problem in that the solid materials caked on the surface of the filter and the dispersion paste was not likely to be effectively filtrated. In this case, the effective filtration means filtration in which only coarse particles in the pigment and lump materials are removed from the paste without variation in basic properties, for example, composition, viscosity, specific gravity, flowability, etc., of the paste for forming thick film, so as to improve cleanliness.